1. (Field of the Invention)
The present invention relates to a two-stroke cycle combustion engine of an air scavenging type that is used as a drive source for a compact work machine such as, for example, a brush cutter.
2. (Description of the Related Art)
The two-stroke cycle combustion engine of an air scavenging type has been well known in the art, in which prior to the combustion chamber being scavenged with an air/fuel mixture, leading scavenging with air is performed to suppress an eventual blow-off of the air/fuel mixture from the exhaust port. The JP Laid-open Patent Publication No. 2004-360656, published Dec. 24, 2004, discloses this type of the combustion engine, in which a pair of first scavenging passage and a pair of second scavenging passages are provided inside a cylinder block and a crankcase so that an air introduced from the outside into an air passage can be once introduced into the second scavenging passages through an introducing passage and, during the scavenging stroke, the air from the second scavenging passages can be supplied into the combustion chamber prior to the supply of the air/fuel mixture from the first scavenging passages to thereby suppress the eventual blow-off of the air/fuel mixture from the exhaust passage. In this known combustion engine, the introducing passage communicating the air passage with the second scavenging passages is formed between a recess, defined in an outer peripheral portion of the cylinder block, and a plate-shaped lid for closing such recess.
The engine cylinder block manufactured by means of casting is apt to have casting burrs around openings such as, for example, an scavenging opening (a scavenging port) and cast mating sites. Such casting burrs are generally removed by means of a complicated and time consuming work of removing the burrs with a cutting tool such as, for example, a file having been inserted into the cylinder block through the recess defined in the cylinder blocks.
On the other hand, the JP Laid-open Patent Publication No. 2008-138602, published Jun. 19, 2008, discloses a different type of two-stroke cycle combustion engine, in which a space communicated with the air passage is defined between an inner surface of an opening, formed in the cylinder block, and an inner surface of a covering member capped into such opening and is divided by a partition segment, formed integrally with the covering, so as to communicate with a second scavenging port of a second scavenging passage for the supply of an air and in which a recess for reserving the air is formed at an outer periphery portion of the reciprocating piston so that such recess is communicated with the second scavenging port upon the ascending motion during the scavenging stroke. In this known combustion engine, since the partition segment for defining the wall surface of a passage for guiding the air towards the second scavenging port is formed integrally with the covering member, the first and second scavenging ports in the cylinder block in a condition before the covering member is fitted to such cylinder block are exposed to the outside through the opening and, therefore, the removal of the burrs in the cylinder block can be easily performed.
However, since the combustion engine disclosed in the above mentioned publication No. 2008-138602 is so designed that the air from the air passage is introduced from the second scavenging port into the second scavenging passage by way of the recess in the outer periphery of the reciprocating piston, the cost of manufacture becomes high as a result of complication in structure. Also, since the amount of the air introduced into the second scavenging passage is determined depending on the shape of the recess in the reciprocating piston which is a relatively large casting, setting and adjustment of the amount of the air so introduced cannot be accomplished easily.